The present invention is a process for producing an acoustically absorbing backing structure for an ultrasound transceiver and the product produced by this process.
Ultrasound imaging devices have become an important part of medical technology. The most commonly familiar applications for these devices are fetal imaging and cardiac imaging. The transceiver of an ultrasound imaging system is typically housed in a probe that is placed over a portion of the imaging subject's body. The transceiver typically includes an array of piezoelectric elements, for producing the ultrasonic waves, supported on some type of backing structure. Two basic approaches that have been proposed are 1) to cast an epoxy loaded with acoustic absorbing and scattering material in place as a liquid on an array surface or 2) to cast the backing structure separately and to attach it to the array.
For the case of a one-dimensional array, the necessary electrical connections can be made from the side. For a two-dimensional ultrasound transceiver array (a "2-D array"), however, the electrical connections are typically routed through the backing structure. The backing structure is, in turn, connected to a connective media such as a flex circuit that electrically connects each piezoelectric element to a driver and receiver. The difficulty of connecting each piezoelectric element to a connective media through the backing array has been a particularly vexing problem confronting those attempting to construct a 2-D array.
Ideally, a backing structure for a 2-D array should perform four essential functions that are potentially in conflict. First, the backing structure should support the array of piezoelectric elements with sufficient rigidity that the elements are not flexed into each other by the application of pressure to the array. Second, the backing structure should acoustically isolate the elements from one another. Third, the backing structure should electrically isolate the elements from one another. Finally, the backing structure should electrically connect each piezoelectric element to a connective media electrode.
One proposed approach to addressing these performance issues is to interpose a prior art resilient, acoustically absorbing, anisotropically electrically conducting layer between an array of electrodes and an array of piezoelectric elements. In conventional interconnect applications, this layer is constructed of some resilient substance (typically silicone) having a multiplicity of fine conducting wires (typical diameter of about 25 .mu.m or larger connecting the top and bottom major surface of the layer.
Unfortunately, silicone is not sufficiently acoustically absorbent to perform well in a backing structure application. Additionally, the conductor pitch of currently available anisotropic layers is on the order of 300 .mu.m, insufficient to form uniform connections with an ultrasound array having a pitch on the order of 300 .mu.m (to form uniform connections the conductor pitch should be one half the element pitch, or about 150 .mu.m). Moreover, the silicone used in anisotropic layers lacks sufficient rigidity to support the elements of a transceiving array in proper alignment.
There is, moreover, a general problem of forming adequate and uniform electrical connections with this type of layer, especially as, through a prospective course of technological development, ultrasound transceiver elements are reduced in size. The wires used in prior art anisotropic material are so fine that each individual wire presents a non-negligible resistance to the electrical signals sent to the elements and produced by the elements. Hence, an element that contacts more fine wires will have a lower conductivity connection with its corresponding connective media electrode. This has the potential for introducing aliasing and/or random unevenness into the electrical transmission through the anisotropic layer.
A number of different approaches have been proposed for a 2-D array backing structure. In U.S. Pat. No. 5,644,085 a method is described in which a substrate is machined to form a multiplicity of vias. The substrate is then coated with conductive material, to form plated vias, and connected with a block of piezoelectric material. The piezoelectric material is machined to form elements, with the kerfs separating the elements machined into the substrate. With this method the bottom of each piezoelectric element is connected with a number of plated vias. Unfortunately, no technique is shown for connecting the top of each element to a ground connector. Although it would be possible to connect each piezoelectric element top to a ground plane (a sheet of conductive material), this solution is not acoustically optimal. Moreover, the great multiplicity of vias shown in the figures will tend to negate the acoustic absorptiveness of the substrate material. Furthermore, the randomness of this type of approach has the potential to introduce a lack of uniformity into the conductivity of the connections formed and the acoustic properties of the backing layer.
Miller et al. U.S. Pat. No. 5,267,221, Greenstein et al. U.S. Pat. No. 5,592,730, and Kunkel, III, U.S. Pat. No. 5,648,942, all appear to show backing layers built up through additive techniques where conductive wires or elements are positively interspersed with acoustically absorbing material. The principal problem with this type of technique is achieving the smallness of scale (@300 .mu.m.times.300 .mu.m elements, or smaller) typically desired for two dimensional arrays. Because of this, there is a problem of forming adequate and uniform electrical connections with this type of layer, especially as, through a prospective course of technological development, ultrasound transceiver elements are reduced in size. Furthermore, it would be difficult constructing a backing structure where all of the conductive elements are properly positioned to align with transceiver elements, using the cumbersome additive construction techniques disclosed.
What is needed but is not yet available is a method of producing an ultrasound array backing structure that is acoustically absorptive and that ensures an electrical connection between each piezoelectric element and its corresponding electrode that has an insignificant electrical resistance while maintaining sufficient acoustic isolation between elements.